This invention relates to a bearing for an exhaust-gas turbocharger with a sleeve which is arranged in a floating manner in a bore of a bearing housing for the radial mounting of a shaft and which is prevented from rotating by means arranged in the bearing housing; the sleeve having inner and outer bearing faces at each of its two axial end regions and having at least two openings provided axially between the end regions.
U.S. Pat. No. 3,811,741 discloses a bearing for an exhaust-gas turbocharger, which has a sleeve arranged in a floating manner in a bore of a bearing housing. A retaining pin is fastened in the bearing housing and radially engages in a sleeve perforation designed as a radial bore. The sleeve contains further perforations, through which lubricant fed to the bearing faces can flow away. These perforations are also designed as bores, and the sleeve as a whole has a comparatively large mass. The retaining pin rests in the associated bore of the sleeve over a relatively large area, with the result that the free movability of the sleeve is appreciably reduced not only in the direction of rotation, but also in other degrees of freedom. This gives rise to difficulties with respect to the vibration behavior and with respect to operating reliability and length of service life at rotational speeds above 150,000 revolutions per minute. Such high rotational speeds are necessary primarily for so-called small exhaust-gas turbochargers which are used for engines with displacements of up to approximately 1.5 liters and a maximum of 2 liters. In addition, in view of the frictional losses at high rotational speeds, the shafts of exhaust-gas turbochargers of this type are made extremely thin and therefore are highly flexible.
Canadian Pat. No. 718,715 discloses a bearing for an exhaust-gas turbocharger, the bearing sleeve of which has teeth at one axial end. Connected to the bearing housing is a pressure disc with teeth which engage the teeth of the bearing sleeve to prevent rotation of the sleeve. Furthermore, the free moveability of the bearing sleeve is also at the very least restricted in other degrees of freedom. Thus, the other two rotational degrees of freedom, specifically about the spatial axes perpendicular to the longitudinal axis, and the translational degree of freedom in the direction of the longitudinal axis, are restricted to an appreciable extent. A high surface pressure is unavoidable at the contact faces of the teeth of the bearing sleeve and the pressure disc. As a result of frictional corrosion, there is a danger, which increases at higher rotational speeds, that the teeth will be destroyed. Free moveability or floating of the sleeve is not satisfactorily achieved. Furthermore, the sleeve contains two small bores in the center for the lubricant and has a comparatively large mass. This previously known exhaust-gas turbocharger is only designed for rotational speeds of up to approximately 80,000 revolutions per minute.
The German reference book Bauelemente der Feinmechanik ("Precision engineering components") by O. Richter and R. V. Voss, Verlag Technik Berlin, 1952, pages 224 to 225, describes wire clips or retaining rings which are arranged in longitudinal grooves of a shaft and which serve to axially retain a component arranged on the shaft. The retaining ring projects beyond the peripheral surface of the shaft, and the component can come to rest with an associated axial end face against the projecting part of the retaining ring. No connection between the component and the shaft is achieved by the retaining ring.
In exhaust-gas turbochargers of this type which operate at high rotational speeds, further difficulties arise as a result of an effect known as "oil whip", which can occur in speed ranges higher than twice the value of the first or second critical rotational speed. This effect causes the shaft to be excited in such a way that the shaft ends execute a second rotary movement about the geometrical axis which is superposed on the shaft rotation. That part of the shaft located between the bearing faces of the bearing bushings is deflected in the opposite direction. This is referred to as the so-called "whiplash effect". If the superposed rotary movement occurs at half the rotational speed of the shaft, metallic contact can take place between the shaft and the bearing bushing, and this can result in complete loss of the supporting capacity of the bearing and in destruction of the bearing. The deformation of the shaft which occurs can lead to unacceptable edge pressures at the ends of the bearing bushings. Floating bearing bushings can each execute slight radial and tilting movements independently of one another. However, in small exhaust-gas turbochargers which are intended to rotate at very high speeds, particularly above 150,000 revolutions per minute, the aforementioned measures are not sufficient to make it possible to obtain an operationally reliable bearing. Exhaust-gas turbochargers of this type have very small rotating masses and at the same time a minimum shaft diameter, and the unbalanced load caused by the deformation of the shaft can amount to a hundred times the value of the mass of the rotor. The resulting sagging of the shaft or "whiplash" effect which then arises can become unacceptably high, with the result that not only can the aforementioned mechanical damage occur, but also a large amount of noise can be generated.
Commonly owned, copending U.S. patent application Ser. No. 001,909 filed Jan. 9, 1987 discloses a bearing in which the sleeve is prevented from rotating by means of a retaining pin, the tip of which rests on the outer face of one of the narrow webs joining the bearing bushings. Because of manufacturing conditions and limitations, difficulties can arise with respect to the arrangement of the retaining pin.